Although digital implementations have replaced most analog circuitry, an analog world necessitates the use of operational amplifiers within an integrated circuit to build the interface between external systems and internal electronic circuitry within a variety of electronic devices from medical instruments, portable phones, notebook computers, cassette tape recorders and battery operated electronic devices, to name a few. Operational amplifiers are used primarily with externally applied feedback in pulse shaping, filtering, signal processing and instrumentation applications. In an effort to prolong life and to reduce size and weight of these electronic devices, the industry trend demands a smaller, lower voltage and power consumption operational amplifier. Operational amplifiers can be characterized by their low cost, ease of use and wide availability and, thus, are in high demand.
The ideal operational amplifier is a voltage controlled voltage source having a differential input and a single-ended output. Ideal operational amplifier characteristics include infinite gain, zero input offset voltage, infinite input impedance, zero output resistance, high bandwidth, high speed, no frequency dependence, no temperature dependence, no distortion, no processing dependence, sufficient output drive capabilities, and low power consumption. Manufacturing processes, however, generate less than ideal operational amplifier characteristics. Thus, it is the job of the circuit designer to optimize one or more characteristics of the actual operational amplifier in an effort to compensate for non-ideal conditions.
Conventional operational amplifier designs include at least two stages: an input stage and an output stage. The input stage, having a non-inverting input and an inverting input, derives the difference between the two inputs. The differential amplifier is one of the most widely used classes of gain stages in analog IC design. As FIG. 1 illustrates, the input stage 10 includes a pair of transistors 12 and 14 configured as differential amplifier having two symmetrical circuit branches, wherein each branch includes a transistor 12 and 14 coupled to one of the input terminals 26 and 28. Additionally, as active loads each branch includes a second transistor 16 and 18 having directly coupled gates. Each source of transistors 16 and 18 are tied to an upper power supply rail 20. Each branch beneath differential transistor pair 12 and 14 is coupled to the source of current-source transistor 30. Current source transistor 30 is biased by a voltage V.sub.bias. Within each branch, the transistor produces a signal proportional to the voltage on the corresponding input terminal 26 and 28. The output 22 and 24 of the input stage 10 is the difference between the signal in each branch 26 and 28 of the differential amplifier. Ideally, the values of corresponding circuit components in the two branches are identical, so that when identical voltages are applied to each input 26 and 28, i.e. a common-mode input voltage, the signals in each branch are also identical and the output of the input stage 10 is zero.
Conventionally, the common-mode input voltage range of a differential gain stage is the maximum range of dc voltage that can be applied, simultaneously, to both inputs without causing the cutoff or saturation of the pair of differential amplifier transistors or the cutoff, saturation, or breakdown of any of the gain stages inside the operational amplifier. A common-mode input voltage which is at or near one of the supply voltages may drive the transistors in the input stage into either a saturation or cutoff condition. This limits the useful range of common-mode input voltages since they must not approach or exceed either of the operational amplifier's supply voltages. A conventional rule of thumb is that the input signal should not come within about 1 volt of either the high or low power supply rails.
In FIG. 1, the lower limit of the input common-mode range is set by the saturation of the current-source transistor 30 having a threshold voltage V.sub.T or the cutoff of gain transistors, 12 and 14. The lower limit occurs when both inputs are lowered, approaching a voltage within the threshold voltage V.sub.T of the lower power supply rail 34 of voltage-V.sub.LL. The upper limit of the common-mode range is set by the saturation of gain transistors, 12 and 14, as both inputs are raised toward the upper power supply rail 20 of voltage +V.sub.HH. Thus, there is normally a high or low-end of the power-supply range, depending upon the polarity of the differential pair transistors 12 and 14, where the differential pair of transistors 12 and 14 are not operable.
Consequently, operational amplifiers of conventional design are limited in range of operable common-mode input voltages. A wide common-mode range, however, is desirable, allowing easy amplifier interface with devices generating input signals at various dc levels. Presently, in single-supply or ground-sensing operational amplifier stages, the range can extend down to the negative power supply rail, -V.sub.LL. Yet, there exists no single pair differential amplifier approach that extends the common-mode range to include both the negative and positive power supply rails, +V.sub.HH and -V.sub.LL, because the threshold voltage of the differential amplifier pair must be reached prior to each transistor becoming conductive.
For this reason, a favored design approach of an input stage within an operational amplifier includes a complementary dual pair of differential amplifiers to compensate for the high or low-end of the power-supply range where one differential pair is operable and the other is not. This complementary dual pair of differential amplifiers has the capability to extend the common-mode range to include both the negative and positive power supply rails, +V.sub.HH and -V.sub.LL. Thus, the amplifier is enabled to have rail-to-rail input capability. More particularly, the amplifier output signal represents the differential input voltage as its common-mode portion travels the full extent of the power-supply range.
An example of such a design is found in U.S. Pat. No. 5,371,474 which describes several embodiments of a differential amplifier having first and second differential portions operating in parallel to provide representative signal amplification across the full power-supply range. As illustrated in FIG. 2, this input stage 40 having a dual differential amplifier pair 50 and 52 offers a solution to the problems faced with the aforementioned single differential amplifier pair input stage. This proposed approach extends the common-mode range to include both the negative and positive power supply rails. The complementary pair of differential amplifiers 50 and 52 are coupled in parallel such that at least one pair is in operation when the common-mode input voltage is at any voltage within the power-supply range.
The first differential amplifier 50 includes a pair of transistors 42 and 44 configured as a differential amplifier having two symmetrical circuit branches, wherein each branch includes a transistor coupled to one of the input terminals 54 and 56. The second differential amplifier 52 includes a pair of transistors 46 and 48 configured as a differential amplifier having two symmetrical circuit branches, wherein each branch includes a transistor coupled to one of the input terminals 54 and 56. One of the differential amplifier pairs 50 is active for input signals 54 and 56 at or near upper power rail voltage +V.sub.HH, and the other differential amplifier pair 52 is active for input signals at or near lower power rail voltage -V.sub.LL. Summing circuit 64 sums the outputs of the two differential amplifier pairs to obtain an output for the input stage 40. For input signals 54 and 56 that are not near either supply voltage, both of the differential amplifiers are active to a varying degree. Additional circuitry, such as current control circuit 62, may be incorporated to provide for a smooth transition between states in which only one or the other of the differential amplifiers 50 and 52 is active as a common-mode input voltage varies from one supply voltage 58 to the other 60. In this way, the common-mode input range is extended to include both power supply voltages 58 and 60.
The first differential portion 50 amplifies a differential input signal by dividing a first tail current I.sub.N into a pair of first main currents, I.sub.1 and I.sub.2, whose difference is representative of the input signal V.sub.1 when its common-mode voltage V.sub.CM is in the intermediate and high-end ranges. The second differential portion 52 operates in a complementary fashion to amplify the input signal by dividing a second tail current I.sub.p into a pair of second main currents, I.sub.3 and I.sub.4, whose difference is representative of the input signal when the common-mode voltage V.sub.CM is in the intermediate and low-end ranges. As a result, the differential amplifier has rail-to-rail input capability.
However, the increased complexity of an op-amp having dual differential amplifier pairs in the input stage potentially decreases the speed of the amplifier and increases the number of errors during the fabrication process. Just as variations in symmetry between branches of a single differential input stage cause an op-amp of conventional design to exhibit a characteristic input offset voltage, an operational amplifier design employing dual input stage differential amplifiers is subject to similar variations in symmetry between branches of each of its differential amplifiers.
Hence, a need exists for a versatile operational amplifier that can be used in a variety of applications powered from battery sources, especially low voltage applications that do not diminish the characteristics of an operational amplifier. A need exists for an operational amplifier input stage that provides high input impedance and a low input offset voltage. A need exists for an operational amplifier that minimizes transistors in the signal path for providing high speed and high bandwidth and still have both input and output rail to rail capabilities. A need exists for a single pair differential gain stage within an operational amplifier capable of a wide common-mode range, inclusive of both negative and positive power supply voltage rails.